PHOSPHORYLATION-DEPHOSPHORYLATION DIFFERENTIALLY AFFECTS ACTIVITIES OF SPLICING FACTOR ASF SF2/

SR proteins are a conserved family of splicing factors that function i n both constitutive and activated splicing. We reported previously tha t phosphorylation of the SR protein ASF/SF2 enhances its interaction w ith the U1 snRNP-specific 70K protein and is required for the protein to function in splicing, while other studies have provided evidence th at subsequent dephosphorylation can also be required for SR protein fu nction, at least in constitutive splicing. We now show that the phosph orylation status of ASF/SF2 can differentially affect several properti es of the protein. In keeping with a dynamic cycle of phosphorylation- dephosphorylation during splicing, ASF/SF2 phosphorylation was found t o affect interaction with several putative protein targets in differen t ways: positively, negatively or not at all. Extending these results, we also show that, in contrast to constitutive splicing, dephosphoryl ation is not required for ASF/SF2 to function as a splicing activator. We discuss these results with respect to the differential protein-pro tein interactions that must occur during constitutive and activated sp licing.